Update Length constraint description (mb_strlen)

Author: guilliamxavier

reference/constraints/Length.rst

@@ -143,9 +143,8 @@ charset
 **type**: ``string``  **default**: ``UTF-8``
 
 The charset to be used when computing value's length. The
-:phpfunction:`grapheme_strlen` PHP function is used if available. If not,
-the :phpfunction:`mb_strlen` PHP function is used if available. If neither
-are available, the :phpfunction:`strlen` PHP function is used.
+:phpfunction:`mb_check_encoding` and :phpfunction:`mb_strlen`
+PHP functions are used.
 
 minMessage
 ~~~~~~~~~~